Field of the Invention
The present invention relates to a heat exchanger, in particular for a condensation boiler.
Background
More specifically, the invention relates to a heat exchanger of the type comprising a helical flow conduit for a liquid, made with a pipe of extruded thermally conductive material, in particular aluminium or an alloy thereof, provided with a pair of facing and essentially mutually parallel fins, which extend longitudinally from a portion of the outer surface thereof, said pipe being helically wound about a longitudinal axis such as to form a sequence of adjacent turns separated by interspaces through which, during use, hot gases, in particular combustion fumes, flow; said fins extending helically, towards the outside with respect to the axis of said helical conduit.
An exchanger of this type is described for example in European patent EP 1,750,070 B1.
These heat exchangers are typically used in boilers, in particular of the wall type, in combination with an internal burner which burns a mixture of air and combustible gas. The hot gases (fumes) generated by the combustion flow over the helical conduit of the heat exchanger and pass through the interspaces between its turns, releasing heat to the liquid (typically water) which circulates inside it.
In order to be able to ensure a high energy efficiency during operation, the turns of the helical conduit of the exchanger must be relatively close to each other.
In condensation boilers, a first portion of an exchanger of the aforementioned type, which extends around the burner, performs the heat exchange between the hot gases generated by combustion and the liquid flowing inside the exchanger, while a second portion performs an additional recovery of heat from the aforementioned combusted gases and also allows recovery of the latent condensation heat of the water vapour generated during combustion.
A problem which affects heat exchangers of the type defined above, in particular in condensation boilers, consists of corrosion phenomena.
The presence, in the fumes, of water vapour, sulphur and NOx results in the formation of sulphuric acid and nitric acid, which are very corrosive and cause the formation of oxides on the surfaces of the exchanger.
This problem is particularly important in the case of exchangers where the helical conduit is made of aluminium or alloys thereof, since aluminium oxides are relatively “voluminous” and their formation may result rapidly in blocking up of the interstices between the turns of the aforementioned exchangers.